The present invention relates in general to circuits for processing capacitive voltages and in particular to a new and useful impedance transformer circuit for capacitive voltage sources having high output impedance.
It is known that capacitive transducers, particularly capacitor microphones, deliver an undistorted output voltage proportional to a diaphragm displacement only if, with the diaphragm (or equivalent structure) being moved, the electric charge applied to the capacitor transducer remains constant. Consequently, it seems imperative to prevent the amplifier or impedance transformer following the capacitive transducer from taking up any current as far as possible. This requirement can be satisfied only with a series connected amplifier or impedance transformer having a very high input impedance and a negligible input capacitance. Further, to prevent parasitic capacitances which may occur between the connections of active circuit elements, from discharging the capacitor transducer, with current flowing through the parasitic capacitors, general compensating measures are taken which are aimed at decidedly minimizing the differential DC voltage between the connections and the signal voltages appearing relative to the reference potential "zero". For this purpose, amplifiers comprising a field-effect transistor in bootstrap connection are commonly used, following the capacitive transducer. Such an amplifier which is intended to be employed for a standard microphone, is described in the manual for the "Artificial Voice" model 4219, by the company Bruel and Kjaer.
If the amplifier or impedance transformer only comprises a FET in sequential source connection, parasitic currents flowing through the parasitic capacitors of the FET in operation withdraw charges from the capacitive transducer, for example a capacitor microphone, with the result that at high sonic pressures, distortions appear in the output voltage. This may be remedied by operating the FET in bootstrap connection which is almost perfectly suited for removing distortion from the output voltage, however, has the substantial disadvantage of introducing a very strong noise into the signal.